Cellulosic ethanol has been pursued as an alternative liquid fuel to petroleum derivatives, due to the environmental impact and non-renewability of current petroleum use. Production of ethanol from lignocellulosic biomass is a potentially carbon-neutral process, wherein any carbon dioxide released during combustion of ethanol would have been released by natural decomposition of the biomass. Furthermore, the bioconversion of lignocellulose to alcohol fuel also allows for the production of value-added biomaterials and biochemicals, such as furfural, acetic acid and lignin.
Enzyme hydrolysis of a cellulosic material results in the release of glucose monomers, which can be fermented and distilled into fuel-grade ethanol. Commercialization of this technology is dependent on high yields from enzymatic hydrolysis, yet hydrolysis yields from native biomass are generally less than 20% of component cellulose. This is due to biomass recalcitrance.
Recalcitrance is defined as the natural resistance of plant material to enzymatic and microbial degradation. It is an inherent feature of fiber wall structure, and is seen from the molecular level (i.e. cellulose strands organized into crystalline domains) up to the cellular level (i.e. cellulose fibrils arranged in layers, with pore space filled by hemicellulose and lignin). The result is restricted access to cellulose surface area and a loss of enzyme activity via non-productive binding with hemicellulose and lignin. Accessibility to cellulose within the pore structure is, in particular, a major factor impacting hydrolysis yields. To counteract this structural resistance to enzymes, pretreatment technology is employed prior to enzymatic hydrolysis.
Pretreatment technology uses chemical or mechanical action to alter biomass structure such that hydrolysis yields are maximized. As cellulose accessibility is lowered by the presence of lignin and hemicellulose, more pretreatment methods utilize chemical fractionation. This generally includes the removal of hemicellulose by acid hydrolysis, or the removal of lignin by alkalines or organic solvents. The fractionated polymers are often a source of value-added products, such as platform chemicals from hemicellulose and adhesives or fuel from lignin. The physical structure will generally be partially disintegrated and size-reduced to increase surface area. Technologies such as steam explosion, dilute acid, and organosolv have been shown to effectively increase hydrolysis yields to the desired level of over 90%.
Pretreatment of wood suffers from two major pitfalls. The first is the high cost of downsizing wood chips to fibers or powder. Most pretreatment methods designed from wood include a size-reduction step prior to chemical treatment to ensure effective cooking and maximum yields from enzyme hydrolysis. Comminution of wet wood consumes a prohibitive amount of energy, accounting for up to 30% of potential process costs for cellulosic ethanol.
Certain pretreatments can avoid the need for comminution, either by explosive depressurization (steam explosion) or the use of solvents (organosolv). However, these pretreatments suffer from the second pitfall, which is a lack of mature equipment and technologies. For both, the large scale reactors needed do not currently exist, thus implementing these pretreatments would be capital intensive. For a pretreatment to be commercially viable in the short-term the equipment used should be well-understood with a developed knowledge base available.